1. Field of the Invention
The present invention relates to a semiconductor device manufactured by using a thin film multilevel interconnection technique, and more specifically, to a multichip module (MCM) manufactured by mounting LSI bare chips on a multilayer substrate.
2. Description of the Related Art
Recently, in accordance with an increase in the operation speed of computers and communication equipments, the problem of time delays due to a long distance between LSIs has become more serious. The LSIs manufactured by the method of mounting each one of a number of LSI chips on a printed circuit board in a package state, cannot exhibit a sufficient performance in order to overcome the problem. A solution to the problem has been proposed in the form of a multichip module (MCM) in which a number of LSI bare chips are mounted on a multilayer substrate. MCMs of this type are discussed in, for example, U.S. Pat. No. 4,692,839, "MULTIPLE CHIP INTERCONNECTION SYSTEM AND PACKAGE" Lee et al., IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS AND MANUFACTURING TECHNOLOGY, VOL. 15, NO. 1, February 1992, pages 7-14, "Microcarrier for LSI Chip Used in the HITAC M-880 Processor Group", Inoue et al., and IEEE/CHMT '91 IEMT Symposium, pages 348-355, "CONCURRENT ENGINEERING REALIZATION THROUGH MULTICHIP MODULE VEHICLES: TECHNOLOGY and CAD TOOL EQUIPMENTS", Wyatt.
Multichip modules are generally categorized into the following types according to the type of substrate used, i.e., for example, MCM-L having a resin-made printed circuit board on which bare chips are directly mounted, MCM-C having a ceramic multilayer substrate prepared by printing interconnections respectively on ceramic green sheets, laminating these green sheets and sintering the laminated green sheets, and MCM-D having a thin film multilayer substrate. Of these types, more attention is paid to the MCM-D since it exhibits a highly-accurate electrical characteristic, high wiring density, and the like.
In the case of MCM-D, a base substrate, which serves as a basic material, is required to form a thin film interconnection. By way of the base substrate, a silicon wafer, a metal plate of aluminum or the like, or a ceramic substrate of alumina, aluminum nitride or the like can be used. In the case where a ceramic substrate is used, interconnection is formed inside the base substrate, and the base substrate can also serve as a package, thus increasing its packaging density. Such a structure is particularly categorized as MCM-D/C.
However, the MCM-D and MCM-D/C types, since both are manufactured by the thin film interconnection technique, each include an insulation film which is very thin, and therefore the mechanical strengths of these types are poor. When bonding an LSI chip and a substrate, the insulation layer situated underneath the pad may become cracked due to the shock of bonding. Particularly, when the wiring layer (or interconnection) is located underneath the pad, the portion of the insulation film situated underneath the pad will not be formed flat and thus the portion is inevitably thin. Consequently, in a practical sense, a wiring layer cannot be provided under a pad, and therefore in the case of a multichip module having a number of wiring layers, the size of the substrate, or the number of wiring layers must be increased.
In the meantime, in accordance with an increase in speed of a signal processing and with a demand for an increase in the capacity of a memory, there is greater need for appropriate control of the characteristic impedance of a signal wiring layer. In the case of the MCM-D/C type, a ground plane can be provided in a ceramic multilayer substrate. However, the contraction of ceramic green sheets, which occurs during sintering, may vary from one sheet to another and accordingly, the distance between each green sheet and the signal wiring layer of a thin film wiring portion may vary from one sheet to another. Consequently, the characteristic impedance cannot be properly controlled. In order to avoid this, a ground plane is, in some cases, provided at the lowermost layer of the thin film wiring portion.
FIG. 1 is a cross sectional view showing a structure of an MCM-D/C of the above-described type. As shown in this figure, a ground plane 51 made of a thin film conductor is formed on a base substrate 50 made of ceramic or the like. A thin film insulator 52 is formed on the ground plane 51, and a thin film wiring 53 is formed on the insulator 52. As mentioned above, the thin film wiring 53 is not formed underneath the bonding pad 54. In general, the thin film insulator 52 is formed to be remarkably thin as compared to a width of the thin film wiring 53. For example, for the width of the thin film wiring 53 of 30 .mu.m, the thickness of the thin film insulator 52 is about 5 to 10 .mu.m. Such a fact indicates that the characteristic impedance of the thin film wiring 53 is lowered. For example, in the case of a general thin film wiring 53 having an insulator made of an polyimide or SiO.sub.2, the characteristic impedance is about 30 to 40 .OMEGA..
An appropriate value for the characteristic impedance may vary in accordance with the type of semiconductor device used; however it is generally said to be 50 to 75 .OMEGA.. In the case where the characteristic impedance is low, the wiring load increases, disabling a high-speed operation. Consequently, a buffer having a high load driving performance must be used, thus increasing its switching noise and power consumption. A solution to this problem has been proposed in the form of a technical idea of increasing the thickness of the thin film insulator 52; however formation of a thick insulation film using the thin film technique, results in a significant increase in production cost and a low yield result, and the technical idea is not completely suitable for preparing a layer which has a sufficient thickness.
As described above, in the conventional multichip module having a thin film multilevel interconnection, the wiring layer may not be situated underneath a bonding pad, and therefore the wiring density is limited. Further, the characteristic impedance of a signal wiring layer is very low especially when the insulation film is made of a thin film insulator, and cannot be properly controlled.